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| Ask about anything online, and you'll soon find a war of words and opinion breaks out. |
So, why is it that there's so much disagreement out there? I just had a look at one thread where supposedly-experienced hams fought it out amongst themselves to see which one could best trash the notion that a vertical delta loop with a base wire at hip height is any good for DX.
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| No good for DX? Not my view. |
It seems this Ministry of Misinformation isn't limited to the forums. There are plenty of web pages that will tell you a 20m loop, for example, can be anything from 20m to 23m long (yes, a whopping 15% variance!), and that the input impedance is anything from 200 Ohms to 100 Ohms - an even bigger disagreement.
Then we have the fights about where to feed the delta, and how that affects its performance. Feed it at the lower corner? Tsch! It'll never work properly and the currents won't work out.
So, despite having a vast number of hams out there, of which quite a few use delta loops, it can be concluded that:
(1) Nobody has a reliable web page that contains an agreed wire length for a delta loop antenna for 20m (or most other bands)
(2) Nobody has a reliable set of actual or modelled results for antenna impedance
(3) Arising from (2), there is no agreement as to whether a delta fed with coax and balun should have a 2:1 or 4:1 fitted.
Not bad for supposedly clever, licensed and experienced people!
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| We can argue all day, but the pole and wire you see behind me has worked all the way around the globe without too much difficulty. Any antenna is better than no antenna remains certainly true, though a delta is not just 'any' antenna. |
(1) A vertically-orientated delta loop, wire length of 21.3m, fed at the bottom corner with a 4:1 balun matches reasonably well via an ATU. It's not ideal and probably has too high a loss level.
(2) Despite (1) above, I've worked around the globe with this arrangement. You can sit there and pontificate and criticise as much as you like, but for most hams, having an acceptable SWR, a simple, effective wire DX antenna at low mounting heights that gets into the Pacific from Europe is their general idea of success.
(3) It's likely the impedance of the delta loop (base wire at 1.2m, apex at about 8m), is lower than what is preached by forum jockeys and experts alike. It seems to be towards the lower, 100 Ohm end of the range than the upper, 200 Ohm. Actual measurement by a colleague on a commercial, apex down aluminium and wire delta shows an impedance of about 87 Ohms.
(4) Leading on from (3), and from reports of the more sensible, helpful hams out there indicates that a 2:1 balun may well give a better match to 50 Ohm coax. I'm trying this out as soon as my 2:1 arrives from the US this week.
(5) Using twin feed gives good matching on 20m-10m, but the 4:1 on the ATU does again leave the matching at a less than ideal point, though it's still less than 1.5:1 on all bands.
I'll let you know how the 2:1 works out later...
4 comments:
Did you ever post your findings on the 2:1 balun with the vertical delta loop? I am about to attempt a similar thing using a Buxcomm balun (looks poorly made and is it really 2:1?). It looks like in later posts you are using the 4:1.
Thanks and 73's, Paul (NT7U)
Hi Paul,
Yes, sorry I didn't update the post!
I bought an expensive 2:1 from the US, as they are not so easy to find in the UK. It didn't work! I tried it on my home and portable deltas, couldn't get a good match!
I went back to the drawing board and re-fitted the 4:1, connected the rig at the antenna, used a good SWR meter and started testing. The wire was too long; I knew that but it was way too long - so I cut it back until it reached about 1:1.6 or so, bringing it in at the theoretical length of about 21.3m; that was good enough for me as I didn't have any spare kevlar wire to make another if I went too far!
At the end of the coax, the match is perfect 1:1, and about 1:1.3 on 10m, both without an ATU.
So there you go. Most of my problem was relying on the SWR meter of the manual ATU (MFJ-941E) to trim the wire, which seems to have something of a mind of its own. Stick a standalone meter and/or use the rig's SWR meter, and they tell you a different story!
Thanks for the reply. I put up the 17m loop and it was also resonating quite low (around 17.8 MHz). This was with a 2:1 current balun (Buxcomm) and insulated stranded copper wire of estimated 16 AWG. I expected it to be low (long) because of the wire insulation and baluns as, from my experience, they seem to give some virtual length.
After pruning it to resonate I get almost no measurable reflected power across all of 17m! I can crank the meter sensitivity to where it would bend the needle on FWD and the reflected is still just moving a bit.
Actually, this is how I remember the 20m delta loop of my youth. It was made with aluminum tubing, had two elements, and I used a gamma match to mate it with coax. SWR was maybe 1.2:1 when you got to the edges of the band but broadly flat with barely measurable reflection also. To me antennas should be resonant if at all possible and behave like that! Receiving, I seem to get more than 2 S-units difference between the side and front of the loop - basically what I expect for a single loop.
I hope to own an antenna analyzer some day soon so I can measure impedance and have high confidence in the matching components. Knowing how to model using EZNEC, or similar, is on the list as well. I'm not sure what a NEC would say but from all the reading I have done a horizontal loop is around 200 ohms and when vertical it is a bit over 100 ohms (110 ohms to 120 ohms). That is what I recall in my dim bulb.
I think a two element phased delta loop would be great. The phasing could be done at the individual loop's natural impedance near 110 ohms. Paralleled I would assume that the two elements would terminate at around 50 ohms. Maybe interaction and phase differences wouldn't make that so but if it were I'm thinking the use of a 1:1 balun to make a clean transition to coax would give great results. I've built it in my head already!
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